A study on solutions for avoiding liquid segregation phenomena in thixoforming process: Part I. Constitutive modeling an
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INTRODUCTION
THE process of forming a partially melted material with an equiaxed microstructure into a net shape part within dies has been termed thixocasting, thixoforging, or, more generally, thixoforming. Thixocasting usually refers to the process of injecting the billet into the die by a ram or plunger as in die casting, and in fact the early work on semisolid materials (SSMs) employed die casting machines and dies. Placing the billet within open dies and squeezing the two halves together is often referred to as thixoforging. However, the deformation and flow of the SSMs within the die is different from that of pure liquid or pure solid in either process, and the term thixoforming is generally preferred. The thixoforming (or SSMs forming), which was originally developed at Massachusetts Institute of Technology (Cambridge, MA) in the 1970s, is now being practiced commercially on the three continents of North America, Europe, and Asia.[1–7] The thixoforming process is one of the new net shape forming processes used to fabricate the final part by loading the materials at a temperature (semisolid state or mushy state) between the liquidus and the solidus. The characteristics of the thixoforming process can decrease segregation because of the improvement in fluidity in a globular microstructure state, and this process uses flow without an air entrapment. Actual trends in the automotive industry lead to an increasing use of lightweight structures imposing the need for high strength aluminum alloys combined with complex shape. Thixoformed products play an imperative role in weight reduction considerations. In a study on the numerical analysis of the thixoforming, Kumar et al.[8] proposed the mathematical model of a flow C.G. KANG, Faculty Member, and H.K. JUNG, Researcher, are with the School of Mechanical Engineering, Engineering Research Center for Net Shape and Die Manufacturing, Pusan National University, Pusan, 609-735, Korea. Manuscript submitted September 10, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS B
behavior of SSMs considering the interaction of particles. Gunasekera[9] published the constitutive equation of SSMs using the geometric shape of a solid particle and of a liquid. Kang et al.[10] verified, from the numerical analysis and experimental results, that the mechanical properties of thixoforged parts using the orifice die shape are superior to those for the straight die shape, for which the final solidification position exists at the middle of the upper and lower die. Jung and Kang[11,12] proposed the optimal coil length through the numerical simulation of the induction heating process based on the effective coil length and an objective function by introducing an optimization technique. The most important factor in the thixoforming analysis is the relationship formulation between the stress and the strain considering the grain size and the deformation of the solid particles with a globular microstructure. No new formulation of the relationship between the stress and the strain needed in a thixofo
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